The present invention relates to improvements in printing machines of the offset type.
Offset printing machines generally comprise a blanket cylinder and a counter cylinder between which a web which is to be printed passes, a plate cylinder tangential to the blanket cylinder, an inking unit and a moistening device ensuring a distribution of a film of ink and a film of moistening liquid on the periphery of the plate cylinder. The inking unit comprises an inking roller having a peripheral coating made of supple material (rubber), this inking roller receiving ink from an inking assembly, via ink vibrator rollers and inking cylinders, and depositing this ink on the plate cylinder. The inking roller is applied to the plate cylinder under a more or less high pressure, producing a more or less considerable crushing of the supple peripheral coating. This crushing is generally constant and the length of the crushing arc may vary for example between 3 and 5 mm. If this crushing is too great, a barrier is created against the moistening liquid as well as trails on the impression, whilst, if the crushing is not great enough, the quantity of ink transmitted to the plate cylinder is not sufficient.
The plate cylinder carries roller tracks which are in contact with corresponding roller tracks fast with the blanket cylinder, with a view to ensuring a constant centre distance between these two cylinders. To ensure a constant pressure contact (crushing of the supple coating of the inking roller) between the inking roller and the plate cylinder, it may also be provided to place roller tracks on the inking roller, these roller tracks abutting on those of the plate cylinder. However, this solution presents a drawback in that, as the printing machine functions, the inking roller wears out, this requiring an adjustment of this inking roller and consequently a reduction of its diameter. Consequently, the roller tracks must be changed, the initial roller tracks having to be replaced by a roller track of smaller diameter.
Furthermore, in known offset printing machines, it is necessary, when the format is changed, to proceed, before a new series of impression in the new format is started, with a preliminary operation consisting in a fresh adjustment of the pressure contact of the inking roller on the plate cylinder in order to obtain a good quality impression. This preliminary operation naturally involves an appreciable loss of time.